Encapsulated propellant agent and method of encapsulation



tent 3,3 75,l4 7 Patented Mar. 26, 1968 tice 3,375,147 ENCAPSULATED PROPELLANT AGENT AND METHOD OF ENCAPSULATION William J. Sparks, Westfield, and Lawrence Spenadel,

Fanwood, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 802,257, Mar. 26, 1959. This application Oct. 9, 1961, Ser. No. 144,285

Claims. (Cl. 1496) The present application is a continuation-in-part of US. application, Ser. No. 802,257, filed Mar. 26, 1959, by William I. Sparks and Lawrence Spenadel, wherein the claims are directed to a solid-liquid rocket propellant wherein solid powdered metal full of high-energy value is compounded with the capsules containing the liquid nitric acid and its mixtures with compatible oxidizing agents of high-energy value encapsulated by polymeric hydrocarbon which is nitrated in forming the capsules. The high-energy oxidizing agents include compounds containing nitro groups, nitrate groups and halogens.

The present invention relates to encapsulating liquids and propellant systems wherein the liquid portion is encapsulated in a polymeric substance. The invention is concerned not only with the method for encapsulating the liquid, which is thereafter compounded with solid materials, but also the novel propellant system obtained.

There is considerable interest in the use of solid propellant systems in rockets because they are less diflicult to handle and the rocket itself is simpler to design and construct. The type of rocket to which the present invention has particular application is the type of rocket propulsion device commonly designated as a pure rocket, that is to say, a thrust producer which does not make use of the surrounding atmosphere. The propellant system in these rockets comprises fuel and oxidizing material which react either spontaneously or upon ignition to produce gases which escape from the combustion chamber through a thrust nozzle to impart thrust to the rocket. Liquid oxidizing agents are generally more effective than their solid counter arts because of their high reactivity. However, considerable ditficulty has been encountered in attempting to combine liquid oxidizing agents with solid fuels because of the tendency of the two materials to prematurely react.

An object of the invention is to provide a new rocket propellant system which utilizes a liquid oxidizing agent and a solid fuel. Another object of the invention is to provide a method for encapsulating liquid oxidizers so as to render them stable in the presence of fuel until ignited.

In accordance with the present invention, a liquid oxidizing agent is encapsulated with a polymeric substance and the capsule is combined with fuel to produce a hybrid propellant system having good stability and high thrust. In carrying out the invention, drops of oxidizing agent pass through a zone containing polymer in a liquid form and the drops coated with a thin film of said polymer are recovered from the encapsulating zone. The encapsulated drops of oxidizing agent may be washed with a liquid which is either a solvent or nonsolvent for the polymeric substance surrounding the drop. The washing removes any excess polymer and reduces its tackiness. While tackiness is not an undesirable property because it assists in bringing the oxidizing agent into closer contact with the fuel, which sticks to the surface of the polymer, it may be desirable in some instances to have a tack-free capsule. The capsule of oxidizing agent, which may vary in diameter from i454" to A", is admixed with a solid fuel, preferably a metal selected from Groups II-A, III-A, lV-A and IV-B of the Periodic Chart of Elements, Langes Handbook of Chemistry, 8th edition, pages 56-57, or metal hydride in a weight ratio of 1 to 2: 1. Suitable fuels include aluminum powder, magnesium powder, boron powder, aluminum hydride, magnesium hydride, titanium hydride and boron hydrides. The two components may be contacted with each other in any suitable manner which may be either batchwise or continuous. It is often advantageous to have between about 0.5 and 50 wt. percent, including the weight of the encapsulating polymer, of a binder present which will maintain the oxidizing agent and fuel in a semi-rigid form which is somewhat flexible and has a tensile strength of at least 50 lbs. p.s.i. The binder, which may be a low molecular weight resin, rubber or asphalt, can be blended with the fuel prior to mixing the fuel with the oxidizing agent. Some examples of suitable binders are isopreneisobutylene butyl rubber, polyisobutylene, polyethylene and polypropylene. In order to obtain very high specific impulses, it is best to use less than about 10 wt. percent of both binder and polymer. The compounded propellant may be molded into any desired shape prior to placing it in the combustion chamber of the rocket.

The oxidizing agent which is a liquid, such as red fuming nitric acid (RFNA) white fuming nitric acid ('WFNA) or concentrated nitric acid, may be sprayed in the form of discrete drops onto the surface of a solution containing from about 10 to wt. percent polymer dissolved therein. If the oxidizing agent does not react with the polymer to form a film around it, it is necessary to add reactive substances to the agent to promote encapsulation. In the case of RFNA, this may be a catalyst or promoter for nitration reactions such as HF, BF acetic anhydride, acetone, sulfur and inorganic salts. It has been noted that the drops are most advantageously encapsulated with a film of polymer when the solution contains about 20 to 30 wt. percent of polymer. Since the thickness of the encapsulating substance will vary according to the contact time and the viscosity of the polymer solution, the encapsulating solution should be adjusted accordingly. In order to obtain capsules having walls of 0.5 to 40 mils thick, the drop should be in contact with the encapsulating solution for 10 seconds to 30 minutes. In order to avoid the presence of an excess amount of polymer in the propellant system, it is advantageous to control the encapsulating reaction to produce capsules which are about of an inch in diameter having walls that are 1 to 5 mils thick.

The oxidizing agent is preferably one which reacts with the polymer to form an insoluble film which surrounds the oxidizing agent. However, any oxidizing agent which is compatible with a nitrating substance, e.g., concentrated nitric acid, may be used in admixture with said nitrating substance. In one embodiment, relatively small drops of an oxidizing agent containing a minor amount of nitrating agent, i.e., about 1 to 20 wt. percent, are contacted with a polymer solution at about P. which is not too viscous to permit the drops of oxidizing agent to pass through it. In many instances the nitrating agent is also an excellent oxidizer, e.g., RFNA. Among the oxidizing agents which may be combined with, say, an inorganic nitrating agent are the nitroparaffins, especially the methane, ethane and propane derivatives, e.g., tetranitromethane, nitromethane, nitroethane and nitropropane. Substituted C to C nitroparafiins, such as brornotrinitromethane, may also be employed. The polymer should contain reactive sites which permit it to combine with the oxidizing agent-nitrating agent mixture and thus form a semi-rigid film around the drop. Suitable polymers include such substances as polybutadiene, styrene-butadiene rubber, polyisobutylene, isobutylene-isoprene rubber (butyl rubber), polyethylene, polypropylene, and styrene-isobutylene polymer. The polymers generally have a hydrocarbon backbone and preferably have a Staudinger molecular Weight of about 10,000 to 150,000. The encapsulating reaction may be carried out from 30 to 125 F. under atmospheric pressure, and for economic reasons it is preferable to employ temperatures of 60 to 90 F. The solvent, which is generally organic, should be inert and capable of dissolving substantial amounts of polymer. As the polymer reacts with the oxidizing agent, it forms an insoluble film around each drop and the coated drops are recovered by any suitable manner, such as filtration or decantation.

Suitable solvents for the encapsulating reaction include r such things as carbon tetrachloride, hexane, heptane, octane, dichloroethane, etc.

Since it may be desirable in some instances to have a strong polymer film around the oxidizing agent which is capable of withstanding conventional handling, it is advantageous to include curingand accelerating agents in the rubber solution so that the polymer film encapsulating the oxidizing agent may be cured either at room temperature or elevated temperatures. For instance, a small amount of sulfur may be compounded with the polymer before it is dissolved in the solvent and treated with the oxidizing agent. Of course, curing agents should only be used where the polymer is capable of being vulcanized, e.g. a polymer having double bonds or active groups. Additional strength may be imparted to the polymer by compounding it with other well known compounding substances before dissolving it in the inert organic solvent. Any excess polymer may be removed from the capsules by washing them with additional solvent. Known solvents, such' as organic alcohols and others, may be used to reduce the tackiness of the capsules.

The following examples are given to more fully illustrate the encapsulating process of the present invention.

EXAMPLE 1 A 25 wt. percent solution of styrene-isobutylene copolymer containing 60 wt. percent styrene and having a Staudinger molecular weight of 40,000 was prepared by shaking the polymer in carbon tetrachloride at room temperature overnight. The polymer solution was placed in a vessel which was 2 ft. high and 1 in. wide and drops of red fuming nitric acid were introduced at the top of the vessel and permitted to fall through the polymer solution to the bottom of the vessel where they were recovered in a bucket. It was noted that a film formed around each'drop as it passed from the top of the polymer solution to the bottom of the vessel. The polymer coated drops were removed from the vessel by raising the bucket and decanting the carbon tetrachloride from the capsules. The recovered capsules were then washed with additional carbon tetrachloride and then with petroleum ether. The dried capsules were less tacky than those recovered from the encapsulation zone prior to washing. One of the capsules was melted on a hot plate and it was noted that it produced red fumes which indicated the presence of active red fuming nitric acid. The remaining capsules were admixed with aluminum powder in a 1:1 weight ratio and it was noted that no reaction took place at room temperature.

EXAMPLE 2 Example 1 is repeated except that a liquid mixture consisting of wt. percent RFNA and 95 wt. percent tetranitromethane is used in place of RFNA.

EXAMPLE 3 Example 1 is repeated except that a polystyrene (100,- 000 M.W.) solution was used in place of the styreneisobutylene copolymer.

EXAMPLE 4 Example 1 is repeated except that styrene-butadiene copolymer (SBR) having a Staudinger molecular Weight of 92,000 was used in place of the styrene-isobutylene copolymer.

Resort may be had to various modifications and variations of the present invention without departing from the spirit of the discovery or the scope of the appended claims.

What is claimed is:

1. A method for encapsulating a liquid oxidizing agent containing nitric acid as a nitrating agent which comprises contacting discrete drops of the liquid oxidizing agent with a hydrocarbon polymer solution so that the nitric acid in said drops reacts with the polymer in solution to form a nitrated hydrocarbon film around the drops, and recovering the thus encapsulated drops of said oxidizing agent.

2. The method according to claim 1 in which the drops of liquid oxidizing agent contain a mixture of nitric acid with a compatible high-energy oxidizing agent.

3. A method for encapsulating liquid drops containing oxidizing agents for solid rocket fuels of the group consisting of aluminum, boron, magnesium, and hydrides thereof which comprises contacting discrete drops of the,

liquid oxidizing agent containing at least 1 wt. percent of nitric acid with a 10 to'75 wt. percent solution of a hydrocarbon polymer at about 30 to 125 F. so that the nitric acid in each of the drops reacts by nitration with the hydrocarbon polymer surrounding each of the drops and makes nitrated polymer form a film around each of the drops, thereafter recovering semirigid capsules from the hydrocarbon polymer solution.

4. The method according to claim 3 in which the drops are dispersed in the polymer solution to form 7 capsules having a diameter of about $4 to 5. The method according to claim 3 in which the nitrated polymer-coated drops are washed with carbon tetrachloride, then with petroleum ether, and subsequently dried to obtain less tacky capsules.

6. The method for encapsulating a high-energy oxidizing agent containing nitric acid which comprises contacting discrete liquid drops of the oxidizing agent mixed with nitricacid with rubbery polymeric hydrocarbon in solution to react the nitric acid in each of the discrete drops with surrounding polymeric hydrocarbon, controlling the size of said drops, contact time and viscosity of the polymer solution to obtain capsules having a diameter of $4 to 4" of the polymeric hydrocarbon nitrated surrounding the drops of oxidizing agent, and separating the thusformed capsulesfrom the solution of the polymeric hydrocarbon.

"7. The method according to claim 6 in which the liquid drops of oxidizing agent contain 1 to 20 wt. percent of nitric acid and to 99 wt. percent of C to C nitroparaffin.

No references cited.

BENJAMIN R. PADGETT, Primary Examiner.

OSCAR R. VERTIZ, L. DEWAYNE RUTLEDGE,

- Examiners. 

1. A METHOD FOR ENCAPSULATING A LIQUID OXIDIZING AGENT CONTAINING NITRIC ACID AS A NITRATING AGENT WHICH COMPRISES CONTACTING DISCRETE DROPS OF THE LIQUID OXIDIZING AGENT WITH A HYDROCARBON POLYMER JSOLUTION SO THAT THE NITRIC ACID IN SAID DROPS REACTS WITH THE POLYMER IN SOLUTION TO FORM A NITRATED HYDROCARBON FILM AROUND THE DROPS, AND RECOVERING THE THUS ENCAPSULATED DROPS OF SAID OXIDIZING AGENT. 